(1) Field of the Invention
This invention relates to a radiographic apparatus for medical or industrial use, for obtaining radiographic images based on radiation detection signals fetched at predetermined sampling time intervals by a signal sampling device from a radiation detecting device as radiation is emitted from a radiation emitting device. More particularly, the invention relates to a technique for fully eliminating time lags, due to the radiation detecting device, of the radiation detection signals taken from the radiation detecting device.
(2) Description of the Related Art
In a medical fluoroscopic apparatus which is a typical example of radiographic apparatus, a flat panel X-ray detector (hereinafter called “FPD” as appropriate) has recently been used as an X-ray detecting device for detecting X-ray penetration images of a patient resulting from X-ray emission from an X-ray tube. The FPD includes numerous semiconductor or other X-ray detecting elements arranged longitudinally and transversely on an X-ray detecting surface.
That is, in the fluoroscopic apparatus, X-ray detection signals for one X-ray image are taken at sampling time intervals from the FPD as a patient is irradiated with X rays from the X-ray tube. The fluoroscopic apparatus is constructed to obtain, based on the X-ray detection signals, an X-ray image corresponding to an X-ray penetration image of the patient for every period between sampling intervals. The use of the FPD is advantageous in terms of apparatus construction and image processing since the FPD is lighter and less prone to complicated detecting distortions than the image intensifier used heretofore.
However, the FPD has a drawback of causing time lags whose adverse influence appears in X-ray images. Specifically, when X-ray detection signals are taken from the FPD at short sampling time intervals, the remainder of a signal not picked up adds to a next X-ray detection signal as a lag-behind part. Thus, where X-ray detection signals for one image are taken from the FPD at 30 sampling intervals per second to create X-ray images for dynamic display, the lag-behind part appears as an after-image on a preceding screen to produce a double image. This results in an inconvenience such as blurring of dynamic images.
U.S. Pat. No. 5,249,123 discloses a proposal to solve the problem of the time lag caused by the FPD in acquiring computer tomographic images (CT images). This proposed technique employs a computation for eliminating a lag-behind part from each of radiation detection signals taken from an FPD at sampling time intervals Δt.
That is, in the above U.S. patent, a lag-behind part included in each of the radiation detection signals taken at the sampling time intervals is assumed due to an impulse response formed of a plurality of exponential functions, and the following equation is used to derive radiation detection signal xk with a lag-behind part removed from radiation detection signal yk:       x    k    =            [                        y          k                -                              ∑                          n              =              1                        N                    ⁢                      {                                          α                n                            ·                              [                                  1                  -                                      exp                    ⁡                                          (                                              T                        n                                            )                                                                      ]                            ·                              exp                ⁡                                  (                                      T                    n                                    )                                            ·                              S                nk                                      }                              ]        /                  ∑                  n          =          1                N            ⁢              β        n            in which Tn=−Δt/τn, Snk=Xk—1+exp(Tn)·Sn(k—1),and βn=αn·[1−exp(Tn)],where Δt: sampling intervals;
k: subscript representing a k-th point of time in a sampling time series;
N: the number of exponential functions with different time constants forming the impulse response;
n: subscript representing one of the exponential functions forming the impulse response;
αn: intensity of exponential function n; and
τn: attenuation time constant of exponential function n.
Inventors herein have tried the computation technique proposed in the above U.S. patent. However, the only result obtained is that the above technique cannot avoid artifacts due to the time lag and satisfactory X-ray images cannot be obtained. It has been confirmed that the time lag due to the FPD is not eliminated.
Further, U.S. Pat. No. 5,517,544 discloses a different proposal to solve the problem of the time lag caused by the FPD in acquiring CT images. This technique assumes a time lag of the FPD to be approximated by one exponential function, and removes a lag-behind part from a radiation detection signal by computation. Inventors herein have carefully reviewed the computation technique proposed in this U.S. patent. It has been found, however, that it is impossible for one exponential function to approximate the time lag of the FPD, and the time lag is not eliminated by this technique, either.